The debate over one-way versus two-way streets has been ongoing for more than half a century in American cities. Counter to prevailing engineering wisdom, a new study finds two-way streets may be more efficient, if one is measuring getting people to their destinations.

Many cities have recognized that two-way streets provide substantial benefits to downtown neighborhoods for a variety of reasons:

Two-way streets are better for local businesses that depend heavily on their visibility to passersby.

Two-way streets have been found to be safer than one-way streets. One-way streets correlate with higher speeds and decreased levels of driver attention. Pedestrians prefer crossing two-way streets since drivers tend to travel more slowly on them, and vehicular conflicts are more predictable.

Two-way streets are much less confusing for downtown visitors than one-way streets. Visitors driving in a two-way grid network can easily approach their destination from any direction.

While there has been much agreement on the economic, safety, and livability benefits of two-way streets, traditionally traffic engineers and transportation planners have felt that one-way streets serve traffic more efficiently by allowing for a higher vehicle moving capacity. However, Vikash Gayah, of Penn State University, argues that the concept of “trip-serving capacity” is a better metric of network efficiency than vehicle moving capacity.

Gayah defines trip-serving capacity as the maximum rate at which people reach their destinations. While current research and conventional wisdom suggest that one-way street networks are more efficient than their two-way counterparts, this study shows that one-way networks are sometimes less efficient because they restrict the rate at which people reach their destinations.

Using the new metric of trip-serving capacity, Gayah compared one-way streets to several types of two-way streets (those with full left-turn lanes, those with left-turn pockets, and those that banned left turns). The study found that for short trip lengths (e.g., in small cities), the additional time spent traversing street grids associated with one-way networks created a lower trip-serving capacity than that of two-way networks. Over longer distances (e.g., in larger cities) one-way streets perform better but never exceed the trip capacity of two-way streets with banned left turns. In other words, the two-way network with banned left turns always has a higher trip-serving capacity even when trips are long.

Gayah encourages urban planners and traffic engineers to examine his trip-serving capacity concept when considering converting to two-way streets. “Since residents prefer two-way street networks for a variety of reasons, converting a one-way street network to a two-way operation can improve both efficiency and livability of cities.”